MRF24XA_15 Datasheet, PDF (121/258 Pages) Microchip Technology – Low-Power, 2.4 GHz ISM-Band IEEE 802.15.4™ RF

4.11 Carrier Sense Multiple Access-
Collision Avoidance (CSMA-CA)
Carrier Sense Multiple Access-Collision Avoidance
(CSMA-CA) is performed before transmitting a packet
to increase the odds that the packet is successfully
received without interference from other transmitting
devices nearby.
When enabled (CSMAEN = 1), the MAC automatically
performs CSMA-CS using the underlying Clear
Channel Assessment (CCA) operation. CSMA-CA is
only performed before transmitting a packet (excluding
ACK packets automatically transmitted during Auto-
Acknowledge) in Packet and Repeater mode.
4.11.1 CSMA-CA CONFIGURATION
Setting the CSMAEN register bit enables CSMA-CA.
CSMA-CA is automatically executed when the TXST
register bit is set before the packet is transmitted.
CSMA-CA is considered part of a transmission
operation and it is aborted when the TXST register bit
is cleared, and not when the RXEN register bit is
cleared.
The following register bits are used in the Configuration
of CSMA-CA:
• CSMAEN
• BOMCNT<2:0>
• BOUNIT<7:0>
• MINBE<3:0>
• MAXBE<3:0>
MRF24XA
4.11.2 CSMA-CA BACK-OFF ALGORITHM
1. Wait a random number of Base time units
between 0 and (2^MINBE-1) * BOUNIT<7:0>.
Note:
If MINBE = 0, the first iteration of the CSMA
algorithm performs a CCA operation imme-
diately without any backoff time.
2. Perform a Clear Channel Assessment (CCA)
operation.
3. If CCA fails, then wait for a random number of
Base time units between 0 and (2^(MINBE+1)-1) *
BOUNIT<7:0>.
4. Repeat above two steps until CCA passes,
incrementing the back-off exponent each time,
until the maximum back-off time becomes
(2^MAXBE-1) * BOUNIT<7:0>, or until the number
of attempts is greater than BOMCNT<2:0>.
5. If CCA is failed, but the number of attempts is
less than BOMCNT<2:0>, keep trying with a
back-off time of (2^MAXBE-1) * BOUNIT<7:0>
Base time units until the number of attempts is
greater than BOMCNT<2:0>, or CCA passes.
6. If CCA still fails, the TX CSMA Error event is
generated.
MRF24XA automatically controls an external LNA if
enabled.
 2015 Microchip Technology Inc.
Preliminary
DS70005023C-page 121